A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). As materials of semiconductor thin films applicable to the transistors, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials.
For example, disclosure is made of a transistor whose active layer is formed using an oxide semiconductor containing In, Ga, and Zn and having an electron carrier concentration of lower than 1018/cm3, and a sputtering method is considered the most suitable as a method for depositing an oxide semiconductor film (see Patent Document 1).